Run AutoMerge checks in package

[GunnarFarneback]

AutoMerge is run on PRs to the General registry in order to determine whether they can be merged automatically (well, yeah, that's what the name means).

However, failures to pass automerge typically requires making package updates, a new commit, and trigger a new registration. This is a relatively slow feedback loop for those automerge checks which can be performed locally by the package. It would be nice if packages could opt in to run applicable checks in their own CI tests by adding something similar to

using RegistryCI
check_if_my_package_is_good_for_automerge()

to their runtests.jl.

[ericphanson]

How should it handle the distinction between new packages and new versions? Simplest would be a keyword argument, but it also could try to inspect the registry or something to see if it is already registered (I guess via RegistryInstances)